Historically, all lighting sources have been self-cooling emitters. The emitting and cooling surfaces are essentially the same surfaces for incandescent, fluorescent, halogen, and arc lamps. In addition, over the years, there have been developments which have reduced the weight of these sources such that greater than 30 lumens per gram output levels have been achieved. A wide range of technology advances have occurred ranging from thin glass or ceramic envelopes to metal/glass joining techniques which have allowed incandescent lamps in particular to reduce weight and costs even though operating temperature can exceed several hundred degrees Celsius. Lighter weight materials means less material costs, lower shipping costs, and reduced weight and costs for the fixtures and luminaires that the bulbs go into. The solid-state lighting industry has taken a much different approach based on high-powered packages with separate heat sinks. Unlike every other light source, solid-state light sources must be designed around heat sinks, fans, and active cooling elements. This creates added cost for the light sources and the fixtures, which use these sources. In addition, because heat sinks and fans must be integrated into the fixtures and luminaires it is very difficult to standardize the light source. The LED manufacturers each have their own standard package but it does not include the heat sink or cooling means and in many cases cannot be mounted without the use of soldering or wirebonding steps. This makes field replaceable light sources difficult as well. While the longer life of solid state light sources does mitigate the need to replace the light source on a regular basis, long life does not address the need for change out due to changing user preference or product updates. The need exists for lightweight solid state light sources which contain not only the means for generating the light and electrical interconnect means to the light but also includes the ability to cool the light. There is a need for lightweight self cooling solid state light sources which address reduce costs and allow for standardization of solid state light sources.
A typical incandescent bulb outputs 1200 lumens and weighs 40 grams, which translates into an output of 30 lumens per gram. A compact fluorescent typically outputs 20 lumens per gram. A typical solid-state light bulb outputs 3 lumens per gram due to the added weight of the heat sink, interconnect package, diffusers and other required elements. This low lumens per gram is mainly driven by the LED (light emitting diode) manufacturers desire to sell packages rather than die (additional levels of electrical interconnect and thermal interfaces) and the heat sink or cooling means weight. As stated earlier, not only does this lead to increased weight, which leads to increased materials costs, but this approach also leads to added shipping costs, and added costs for the fixtures and luminaires. In some cases, additional safety supports are required to hold up the heat sinks for solid-state light sources in overhead applications. In other cases, active cooling means have been required which negate the life benefits of solid-state lighting simply to reduce the weight of the heat sinks. The need exists for lightweight solid-state light sources, which relieve the requirement for cooling means by the fixture manufacturers.
Solid state lighting typically consists of a light emitting diode outputting in the UV/Blue wavelengths covered by a luminescent powder dispersed within an organic matrix, such as silicone, as shown in Cao U.S. Pat. No. 6,634,770. As disclosed in Cao, additional heat sinking means cool the light emitting diode. But the phosphor powder is essentially thermally isolated within a low thermal conductivity matrix material. As the efficiency of the light emitting diodes have approached 60% or higher, more than 50% of the heat generated within the solid state light source is being generated within the phosphor powder. This heat generation leads to efficiency drops due to thermal quenching of the phosphor powder. The elevated temperature occurs because the phosphor powder is encased in a low thermal conductivity organic matrix such as silicone (thermal conductivity of 0.1 W/m/K). Even though the typical luminescent powder has a thermal conductivity within the particle itself greater than 10 W/m/K, the composite has an effective thermal conductivity which is dominated by the matrix thermal conductivity which is typically 100 times lower than the particle thermal conductivity due the lack of thermal contact between the phosphor particles Essentially the phosphor powder is encased in a thermally insulative matrix which elevates the temperature of the phosphor powder to greater than 150 degrees C. The heating is due to the stokes shift losses and the less than perfect conversion efficiency of the phosphor powders as they convert shorter wavelength light to longer wavelength light. What is required is a means of conducting the heat out of the phosphor powder while maintaining a reasonable level of transparency.
In Mesli's paper, “Improvement of Ultra High Brightness White LEDs from Global Light Industries”, Proc. SPIE 6797, Manufacturing LEDs for Lighting and Displays, (2007) pp 67970N1 to 67970N9, a temperature of greater than 200 degrees C. was measured for the phosphor powders based on a single high powered LED with 550 mW of blue excitation optical output. This reduced the luminous efficiency by almost 30% and had detrimental effects on the silicone matrix. The need exists for efficient thermally conductive luminescent elements, which can remove the heat, generated within the phosphor powders due to Stokes Shift losses, scatter losses, quantum efficiency losses, and absorption losses. The powder phosphor particles in the Mesli paper are the highest temperature points in the LED device. As LED efficiency improves, this problem only becomes worse as higher blue flux densities become possible. In general, the limiting factor in overall device performance is becoming the operating temperature of the phosphor powders rather than the LED die itself.
The thermal conductivity of a composite material can be modeled to derive an effective thermal conductivity. In most cases the effective thermal conductivity is linearly related to the matrix conductivity. The effective thermal conductivity of the composite is also based on the thermal interface between the filler and the matrix materials. As stated previously, organic matrix materials used in typical solid-state lighting are silicones or epoxies, which have thermal conductivities of approximately 0.1 W/m/K. Therefore the effective thermal conductivity of the luminescent coatings used is much less than 1 W/m/K due to thermal insulative nature of the matrix surrounding the luminescent particles.
Very high loading levels of the luminescent materials can increase the effective thermal conductivity but these loading levels lead to higher optical scatter losses because the organic matrix materials typically have refractive indices around 1.5 versus phosphors, which have refractive indices of 1.8 to 2. Multi-particle scattering as modeled by Griffith in “Scattering of Ultraviolet Radiation in Turbid Suspensions”, J. Appl. Phys. 81 (6), 15 Mar. 1997, pp 2538 to 2546, are strongly influenced by volume loading levels are related the square of the refractive index difference between the filler and the matrix and filler particle size. A large refractive index difference decreases the transmission, especially for thick elements, due to scattering losses. High thermal conductivity fillers also exhibit high refractive index and high volume loading of these fillers will also increase scatter losses. As stated earlier, both thermal conductivity and translucency are required to make a useful self-cooling solid-state light source. If the scatter is too high, the light generated cannot be uniformly distributed and the conversion efficiency drops dramatically. Essentially, the light within the source is trapped within luminescent element rather than being emitted. High thermal conductivity is needed to efficiently transport the heat generated within the LEDs and the luminescent elements out over a large enough surface area such that a reasonable operational temperature can be maintained for all the components in the light source. The need therefore exists for novel luminescent materials, which exhibit high thermal conductivity while maintaining low optical absorption and optical scatter losses.
The amount of heat that can be removed off a surface can be modeled as being directly proportional to the area of the surface, the temperature difference between the surface and the surrounding ambient, and the natural convection coefficient of the surface. Incandescent light sources can have surface temperatures in excess of 150 degrees C. and halogens can exceed 250 degrees C. These allows for very efficient convective and even radiative cooling of these sources. LEDs and their associated phosphor converters tend to prefer temperature under 150 degrees C. Thermal droop adversely effects LED efficiency and thermal quenching can dramatically reduce conversion efficiency in even robust phosphors like CeYag at temperatures over 150 degrees C. Approximately 20% of the heat transfer from a surface at 150 degrees C. can be via radiation with the remainder via natural convection if the ambient is close to 25 degrees C. For a given ambient condition the natural convection coefficient can only be increased slightly using induced draft cooling techniques. Therefore to effectively cool a solid-state light source the surface temperature and surface area needs to be maximized. This is best accomplished by reducing the thermal resistance between the heat sources and the cooling surface of the device. This filing discloses methods and materials, which allow for low thermal resistance designs while maintaining low optical absorption and optical scatter losses.